1. Field of the Invention
The present invention relates to methods for condensing vapor to concentrate naturally-occurring radiation, and to identify and characterize steam and condensable gases in subsurface formations in the ground by observing the increased radiation.
2. History of the Prior Art
In the drilling and operation of oil wells, large amounts of natural gas are used to make steam which reduces the viscosity of the oil. Because of the cost of the natural gas consumed, it is desirable to optimize the placement of steam within the well and observation of the manner in which the steam moves through reservoirs under the ground.
It is extremely difficult, given present technology, to accurately evaluate oil recovery and steam placement from producing oil wells. For this reason, it is common practice to drill observation wells relatively close to the producing wells. The observation wells are not used to produce any oil but rather to evaluate conditions close to the producing wells so that oil recovery and steam placement can be predicted for the adjacent producing wells. The problem is that the conditions belowground can vary considerably between the observation wells and the producing wells even though they may be relatively close to each other. In a heterogeneous belowground reservoir, conditions at the nearby producing well may differ significantly from those at an observation well. At the same time, it is very difficult to accurately evaluate oil recovery and steam placement from the producing wells.
When steam is injected into a well, it accumulates in the underground reservoirs within the well where the oil is located. Consequently, detection of the steam within the well identifies the locations of the oil deposits. Steam and other condensable gases produce detectable radiation because they are carriers of radon gas. Such gamma radiation is detected to identify the location of the steam.
U.S. Pat. No. 2,197,453 of Hassler provides examples of technologies used in the oil industry in which radioactive gaseous emanation (radon) is measured by a gamma detector. Further examples are provided by U.S. Pat. No. 2,385,378 of Piety. The Piety patent is exemplary of technologies in the industry in which the radioactive radon is artificially added.
Russian Patent RU2079650 C1 of Filippov describes the surveying of production reservoirs perforated by cased and uncased holes. The patent describes detection of water-saturated and oil-saturated seams in a perforated reservoir, and involves determination of depth of penetration of a radioactive radon-containing indicator liquid and the degree of change in time of gamma radiation activity in seams.
Still other arrangements which utilize the detection or measurement of the radiation of radon include U.S. Pat. No. 4,221,482 of Macourt which carries out mineral prospecting by detection of radon or iodine, U.S. Pat. No. 4,495,142 of Nakayama which describes a system for monitoring the state of a nuclear reactor core, and U.S. Pat. No. 5,501,099 of Whorff which describes a vapor density measurement system.
Because the data provided by observation wells is of limited accuracy, it would be desirable to be able to optimize the placement and observation of steam in the producing wells themselves. However, this is very difficult to do inasmuch as these wells are typically large-diameter wells, with air-liquid or steam-liquid contacts within a completed interval therein. Modern thermal and epithermal neutron tools for logging to identify steam do not have the depth of investigation to work well. Such tools are designed to make observations from liquid-filled wells.
Accordingly, the need exists to provide methods whereby the location of steam within a producing well can be accurately determined.